DNA branch migration is an important step in both homologous recombination and site-specific recombination. Migration of the Holliday junction generated y homologous recombination can increase or decrease the amount of genetic information that is exchanged between homologues. Although there has been recent progress in determining the structure of Holliday junctions, less is known about the dynamics of branch migration particularly as it relates to the formation of heteroduplex DNA during recombination. As a first step in studying this problem, we have developed a model four-stranded system to systematically examine the effect of mispaired or unpaired bases on nonenzymatic branch migration. Using a series of short duplexes 40-60 bp long, we observe that the reaction consists of several distinct steps, annealing of the two duplexes, initiation of a Holliday junction and branch migration. Our results demonstrate that a single mismatched base pair impedes branch migration. Our results also indicate that the block on branch migration imposed by mismatches is more pronounced in the presence of Mg2+. This probably reflects the difficulty in accommodating non-Watson-Crick base pairs in the highly constrained conformation imposed on four-way DNA junctions in the presence of metal ions. We are also attempting to obtain a chicken cDNA clone for a type I DNA ligase. We wish to exploit the fact that chicken B cells carry out homologous recombination at unusually high frequencies to create a ligase- deficient chicken B cell line by gene targeting. In this way, we can assess the physiological function of type I ligases in these cells.